"If there was even a 1 percent chance of terrorists getting a weapon of mass destruction -- and there has been a small probability of such an occurrence for some time -- the United States must now act as if it were a certainty."This is a ludicrous idea. If we think that some event has a 1% chance of happening, we should simply treat it as if it has a 1% chance of happening. Proper evaluation of risk demands our best estimates of probability and damage, and artificially inflating our estimate from 1% to 100% accomplishes nothing.
I think I understand the intuition behind the One Percent Doctrine: the idea is that some events are so disastrous that they should dominate our decision-making even when they are only expected to occur with low probability. As I mentioned in my post, this is certainly true of all-out nuclear war, which entails the destruction of human life on Earth as we know it. Even here, however, I don't advocate overstating our probability estimates; I simply think that a 1% probability of nuclear winter is intolerable on its own terms.
Meanwhile, I don't think that "terrorists getting a weapon of mass destruction" is nearly enough of a catastrophe that a 1% chance should always shape our policy decisions. Although there's a natural tendency to shrug off the difference between large disasters and overwhelmingly large disasters, this distinction is actually critical. While a few weapons of mass destruction in the hands of terrorists might result in millions of deaths and some cities destroyed, an exchange of thousands of nuclear warheads would cause billions of deaths and the destruction of almost the entire world. To be sure, both would be cataclysmic, but the latter is at least a thousand times worse than the former.
In fact, there is a good case to be made that a 1% probability of a million deaths in a terrorist attack should be no more significant than many of the other considerations involved in foreign policy. After all, if there is a 1% chance of a million people dying, then the "average" number of deaths is 10,000. This is well within an order of magnitude of the number of troop deaths in the Iraq and Afghanistan campaigns, and it's far below the total number of fatalities believed to be a result of the Iraq war. Granted, our analysis of unlikely catastrophes should be more sophisticated than a simple probability-weighted average of deaths, but I think that such a calculation does provide a good starting point for estimating magnitude. The risk of terrorist attacks isn't grave enough that we should roam the world in quixotic pursuit of whatever "threats" appear on the horizon.